MSOs (multi-service operators) provide several services to end users through a fiber optic network, with the final connection to the user through a coaxial connection. The services provided by the MSO typically include broadcast analog video and narrow cast digital services, such as data, VoIP, subscription, pay per view and video on demand (VOD) services. The services are generally allocated a portion of an optical channel, which typically has approximately 1 GHz bandwidth available. While the bandwidth of a channel is generally constrained by the network (including, e.g., the analog optical transmitters and receivers, and the coaxial connection), the number of users connected to the network continues to increase, which together with the new broadband services require increased demand for bandwidth for the desired services.
In recent years wavelength division multiplexed (WDM) optical transmission systems have been increasingly deployed in optical networks to meet the increased demand for bandwidth by providing more than one optical channel over the same optical fiber. The WDM techniques include coarse wavelength division multiplexed (CWDM) and dense wavelength division multiplexed (DWDM) systems. Whether a system is considered to be CWDM or DWDM simply depends upon the optical frequency spacing of the channels utilized in the system.
FIGS. 3 and 4 show simplified block diagrams of conventional WDM transmission arrangements. As illustrated in FIG. 3, data or other information-bearing signals S1, S2, S3 and S4 are respectively applied to the inputs of modulators 2101, 2102, 2103, and 2104. The modulators 2101, 2102, 2103, and 2104, in turn, drive lasers 2121, 2122, 2123, and 2124, respectively. The lasers 2121, 2122, 2123, and 2124 generate data modulated optical channels at wavelengths λ1, λ2, λ3 and λ4, respectively, where λ4>λ3>λ2>λ1. A wavelength division multiplexer (WDM) 214 receives the optical channels and combines them to form a WDM optical signal that is then forwarded onto a single optical transmission path 240.
As illustrated in FIG. 4, digital signals, which may consist of broadcast and narrowcast signals, may be RF frequency multiplexed into the signal band. The digital signals are normally much lower in amplitude than broadcast analog video signals. The arrangement of sending the same broadcast signal and different narrowcast signals over multiple wavelengths (WDM) is a means of providing more segmentation in an optical network. As illustrated in FIG. 4, an RF splitter 216 splits the broadcast signal among the lasers 2121, 2122, 2123, and 2124. As shown, the lasers 212 each receive a different narrowcast signal. The wavelengths carrying the combined broadcast and individual narrow cast signals, λ1, λ2, λ3 and λ4, respectively, are optically multiplexed onto optical fiber 240.
Although WDM optical transmission systems have increased the speed and capacity of optical networks, the performance of such systems is limited by various factors such as chromatic dispersion and the fiber nonlinearity, which can cause, for example, pulse shape change in the case of baseband digital signals and distortions in case of analog signals. These impairments degrade the quality of the optically transmitted information. Fiber nonlinearities, for example, can give rise to crosstalk between optical signals operating at different wavelengths. When crosstalk occurs, modulation components of one signal are superimposed on another signal at a different wavelength. If the level of crosstalk is sufficiently large it will corrupt the information being transmitted by the optical signals impacted by this impairment.
One common cause of crosstalk, in an optical fiber communication system with multiple wavelengths, is Raman scattering. This type of crosstalk is caused by stimulated Raman scattering (SRS) in silica fibers (and other materials) when a pump wave co-propagates with a signal wave through the same fiber. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e. optical phonons) FIG. (1) is a schematic diagram of the stimulated Raman scattering process. FIG. 1 illustrates a pump photon scattering in the Raman media. As a result of the scattering event the pump photon is annihilated and a new signal photon at the Stokes frequency is created along with an optical phonon at the Stokes shift frequency. Both energy and momentum are conserved:ωpump=ωsignal+ωOp phonon and {right arrow over (k)}pump={right arrow over (k)}signal+{right arrow over (k)}Op phonon,  (1)where ωx is the frequency of x and kx is the associated wavevector of x and  is Planck's constant divided by 2π.
FIG. 2 shows how the transfer of energy from Raman gain gives rise to crosstalk. FIG. 2 is a simplified illustration that is useful in facilitating an understanding of Raman crosstalk between two optical channels or signals Si and Sj, where Sj is at a longer wavelength than Si. FIG. 2A shows the signal Si and FIG. 2B shows the signal Sj. For simplicity of illustration Sj is shown as a signal with constant amplitude (i.e. a continuous string of zeros or ones in the case of baseband digital modulation). As indicated in FIG. 2C, the pattern of signal Si (dashed line) is impressed on the signal Sj by the process of Raman amplification. In other words, signal Sj now includes as one of its components the pattern of signal Si. Likewise, since signal Si is pumping the signal Sj, the pattern of signal Sj (had it been modulated) would be impressed upon the pump Si by the process of pump depletion.
In addition to the generation of unwanted crosstalk the SRS process can also lead to the generation of Raman induced second order (CSO: composite second order) and third order (CTB: composite triple beat) distortions. These distortions occur as result of the nonlinear nature of the Raman amplification process which, in the undepleted regime, is exponential in form.
Further, the Raman induced crosstalk and nonlinear distortions are more pronounced when the wavelengths are located near the zero dispersion wavelength of the optical transmission media through which the signals are co-propagating (i.e. the optical fiber). In the case of a near zero dispersion system the optical pump and signal waves are propagating at nearly identical group velocities through the media. The zero dispersion wavelength of a transmission media refers to the wavelength at which an optical signal will have no change in (inverse) group velocity with respect to changes in its optical frequency. The zero dispersion wavelength differs for different transmission media. In this case, the relative positions of the waves with respect to one another will remain nearly fixed throughout the length of the transmission media. Thus, if the signals Si and Sj are at or near the zero dispersion wavelength, they will largely maintain their relative phase with respect to one another. Hence, with very little walk-off occurring between the optical channels the Raman induced crosstalk and distortions can build up along the fiber in a constructive manner. The dispersion will generally increase as the wavelength difference between the optical signal wavelength and the zero dispersion wavelength increases. If the signals Si and Sj are located at wavelengths far displaced from the zero dispersion wavelength, their relative phases will change as they propagate down the transmission path. The levels of Raman induced crosstalk and distortions are much lower in the nonzero dispersion scenario because, as the signals walk away from one another, it becomes more difficult for the crosstalk and distortions to build up constructively along the fiber length.
With reference again to FIGS. 3 and 4, Raman crosstalk may occur among the optical channels λ1, λ2, λ3 and λ4. Raman interactions cause both crosstalk and second order distortions (third order distortions are generally smaller than second order distortions) on each optical channel. In general, the analog signal formats are typically more susceptible to impairment corruption than the digital signal formats.
The widely used optical wavelengths in a typical CATV application, e.g. around 1310 nm, exhibit little relative dispersion between adjacent ITU frequencies, and hence are particularly affected by the SRS and some other fiber nonlinearity effects when used in a WDM system. Moreover, a typical CATV application also uses a single laser to transmit both the broadcast signal and the narrowcast signal. As a result, in order to meet the increased demand for additional bandwidth, CATV MSOs may be required to install more optical fiber to carry additional channels, and then segment their subscriber base between the newly installed optical fiber and the existing fiber. However, this approach requires a significant capital investment for the MSOs and often negotiation of additional access rights to install the optical fiber. Alternatively, the CATV MSOs may use WDM technologies. In order to implement WDM technologies the signal degradations caused by the fiber nonlinearities, such as Raman crosstalk, need to be overcome within the traditional wavelength window, around 1310 nm. The MSO's may use other wavelengths which are less affected by SRS and the other fiber nonlinearities, e.g. 1550 nm. However use of these wavelengths require more expensive optical components, e.g. lasers, dispersion compensators and nodes. This also requires significant changes to their existing optical networks, in addition to a significant capital investment.
Accordingly, it is desirable to have a method and apparatus for reducing the levels of Raman induced crosstalk and distortions that arises among the individual channels comprising a WDM optical system. This is particularly true in the case of a system utilizing optical channels that are located near the zero dispersion wavelength of the transmission medium, and also, in the case of wavelength division multiplexed systems, channels that are operated away from the zero dispersion wavelength. The method and apparatus described herein utilizes destructive interference amongst coherently related signals. In this case the coherence comes about as result of identical modulation information being imparted upon the various optical carriers in the system.